发送自定义消息的时候，extar 类型是空，会有异常

[coder-wusichao]

实例化 JPushMessage 的时候，extar 类型是空，会有异常

case 'receivePushMsg':
{
var map = call.arguments;
var extras = map['extras'];
if (extras != null) {
try {
if (extras is String) {
extras = json.decode(extras);
}
} catch (e) {}
}
_recvCustomMsgController.add(new JPushMessage(
title: map['title'],
message: map['message'] ?? map['content'],
contentType: map['contentType'] ?? map['content_type'],
extras: extras));
}
break;

JPushMessage({this.message, this.contentType, this.title, this.extras});